surrounding island

surround.java

class Solution {
    int n,m;
    public void solve(char[][] board) {
        n= board.length;
        if(n==0)return;
        m=board[0].length;
        for(int i=0;i<n;i++){
            for(int j=0;j<m;j++){
                if(board[i][j]=='X')
                dfsmark(board,i,j);
            }
        }
    }
    
    public void dfsmark(char[][] board,int i,int j){
        if(i<0||j<0||i>=n||j>=m)return;
        if(board[i][j]=='O'){
        if(i!=0&&j!=0&&i!=n&&j!=m)
            board[i][j]='X';
        else
            return;
        }
        if(board[i][j]=='X')
        {
            if(i+1<n)
                dfsmark(board,i+1,j);
            if(i-1>0)
                dfsmark(board,i-1,j);
            if(j+1<m)
                dfsmark(board,i,j+1);
            if(j-1>0)
                dfsmark(board,i,j-1);
        }
    }
}